The present invention relates generally to the operation of fluid piping systems. More particularly, the present invention relates to keeping fluid piping systems free of air and other gases.
Since the advent of commercial nuclear power in the late 1960's, the industry has been aware of issues regarding the accumulation of air and other gases in the high points in various safety-related fluid systems. These systems are designed to prevent nuclear fuel damage given various postulated accident scenarios. Air and gas accumulation in these fluid systems could result in failure of these systems and, in their failure, in turn failure to prevent that fuel damage.
Among the many different manufacturers and different designs of nuclear power plants, there are substantial commonalities. One of those commonalities is the need to eliminate air accumulation from within the safety-related fluid systems. The problem is common to Pressurized Water Reactors (PWRs), Boiling Water Reactors (BWRs), Small Modular Reactors (SMRs), Pebble Bed Modular Reactors (PBMRs), and International Reactor Innovative and Secure reactors (IRIS) and all other designs.
It is common knowledge that in 2011 the damage to reactors at Japan's Fukushima Daiichi power station resulted from a Loss Of Coolant Accident (LOCA). A tsunami knocked out electric power to the plant and also flooded the backup generators needed to run the pumps that cool the reactor core. The pumps failed to operate as expected.
What is less common knowledge is that gas voids in the cooling systems in every existing nuclear plant could also cause those reactor coolant pumps to fail in a similar manner. When a gas void is introduced to a high speed pump, it creates a cavitation shock wave that can destroy the pump and damage instrumentation.
The operators of nuclear power plants are required to demonstrate that they have suitable design, operational, and testing control measures in place for complying with regulations that require, by federal law, these fluid systems to be “full”, i.e. devoid of air and/or gases.
Currently in the nuclear industry, the common ways to detect unwanted air in piping system are to perform ultrasonic test (UT) examinations and to periodically vent suspect locations without knowing whether air has accumulated in those locations of the system or not. These solutions are unsatisfactory because they require radiation exposure of workers and/or the release of potentially contaminated liquids when there may not have been a need for testing or venting. Also, UT probes, in general, cannot remain connected to the piping system in question due the temperature limitations of the UT equipment and associated coupling material. As a consequence, they must be connected each time a UT examination is to be performed, thus taking additional time and resulting in additional exposure of workers to radiation.
Additionally, ultrasonic testing is both time-consuming and expensive for the utility. In the US, each high point must be tested every 30 days. Each test requires a pre-job briefing, system tag-out procedures, dress-out, ingress time, scaffold building around sensitive equipment, time to perform the ultrasonic test, removal of the scaffold, lifting the system tag-out, and egress time. These tasks are all highly procedural and many are performed in a radioactive environment. Additionally, the logistics required to support these efforts and the costs charged to the utility that result from radiation exposure are substantial. Since the formal identification of the problem by the Nuclear Regulatory Commission (NRC) in 2008, there are have been numerous “near miss” events involving gas voids in power plants in the US. Many events result from human error when using ultrasonic testing. Events involving gas voids include Ft. Calhoun (NRC Event 45970), Turkey Point (NRC Event 45971), Dresden (NRC Event 45844), Wolf Creek (NRC Event 45985), Comanche Peak (NRC Event 46786), Kewaunee (NRC Event 48051) among others.
Utilities are required to test each location every 30 days. The existing requirements allow for a dangerous gas void to exist for up to 29 of those 30 days.
The magnitude of the problem combined with the complication and expense of ultrasonic testing has led to an enormous problem for plant operators and owners.
Each existing piping system has a unique configuration of pipe length, pipe volume, system geometry, drag coefficients, pressures and temperatures such that each system would have its own unique frequency of gas accumulation—if only from evaporation. Monthly monitoring by ultrasonic testing does not allow adequate trending frequency analysis to determine if gas accumulation is a result of natural air/gas accumulation or an air leak from a valve within the system.
Currently, outside of the nuclear industry, there are a number of solutions for measuring gas accumulation in piping, for indicating the extent of the gas accumulation, and for venting the accumulated gas. However, these solutions use materials and construction practices that fail to meet the needs of the highly specialized requirements of the commercial nuclear industry, particularly if a device is to penetrate the pressure boundary of fluid piping systems. Additionally, none of the existing methods provides a means of instantly isolating the gas as it accumulates while still allowing for routine maintenance and inspections while the supported cooling system is still in full operation. None of these systems provides a means of determining the natural frequency of gas build-up within the system. With data available on only a monthly basis, trending is difficult. When gas is discovered, root cause analysis becomes difficult or even impossible. The station may discover a gas void that was caused by a single event in the previous 29 days or that the frequency of the accumulation may have changed dramatically due to a leak. Regardless, with the existing state of the art, trending analysis is hindered significantly.
Additional designs are shown and described in WO2013075056 and U.S. Pat. No. 8,505,568, which are incorporated herein in their entirety by reference.